The present invention is in the field of ultrasonic liquid level sensors mounted on the outside of a tank or vessel to measure liquid level at single or multi-point levels or on a continuous basis by the use of sensing flexural waves in the vessel wall.
Measurement of liquid levels in a tank or other vessel by use of an ultrasonic transducer is a well known art. Many instruments of this type have the transducer in contact with the liquid in the vessel interior to sense the level. Another type of ultrasonic liquid level detection instrumentation mounts the transducer on the outside of the vessel wall and transmits the ultrasonic energy to or through the vessel wall.
For example, U.S. Pat. No. 5,663,505, which is assigned to the assignee of the subject application, mounts the transducer on the outside of the vessel wall. Ultrasonic energy is transmitted through the wall and is reflected back through the wall to the transducer. This arrangement operates effectively so long as the ultrasonic energy transmitted through the wall is reflected back from the other side of the inner wall. Due to the nature of the different liquid""s density and vessel wall thicknesses the ultrasonic energy can be attenuated by an amount such that there may be no alarm or a false alarm produced.
U.S. Pat. No. 5,456,114 discloses a system which uses a transmitting mechanical transducer and a receiving mechanical transducer mounted on the outside of a tank/vessel wall to measure liquid level by sensing an elastic wave produced un the vessel wall. Here, the transmitting transducer produces energy to propagate a relative high frequency (12.5 khz) elastic wave through the wall of the vessel. The speed of propagation of the elastic wave through the wall of the tank is effected by whether there is a liquid present against the inner wall of the tank along the propagation path. The change in time of propagation is measured by a zero crossing technique to determine presence or absence of a liquid in contact with the inner wall of the vessel.
This technique has limitations as follows:
(a) any build up of any kind, solid or semi solid, on the vessel inner wall along the propagation path of the elastic wave, will change the propagation time, resulting in false alarm such that the system becomes unreliable.
(b) this liquid level sensing technique requires initial measurement of vessel wall thickness and material since this has a direct effect on the propagation time of the elastic wave. This requires extra effort before installing the system.
(c) the system requires initial calibration as the transmission of the elastic wave at 12.5 kHz provides a very small change in travel time of the elastic wave, e.g., less than 10-15 microseconds. Also, the spacing between the transmitting transducer and receiving transducer is very critical. For example, in a vessel having a steel wall xc2xcxe2x80x3 thick, the propagation of the elastic wave is approximately 30 microseconds/inch. A change in spacing of the transducers of xc2xcxe2x80x3 produces a delay of 7-8 microseconds which is greater than the threshold set by (10 microseconds). Thus, the system of this patent can produce false alarms.
The system disclosed in U.S. Pat. No. 6,192,751 is intended to overcome the problem of solid/semi solid build up on the inner wall of the tank/vessel that is present in the foregoing U.S. Pat. No. 5,456,114. U.S. Pat. No. 6,192,751 discloses a non-invasive level system using a low frequency elastic wave in the vessel wall at a frequency of 750 Hz. This system uses a mechanical shaker, disclosed as Ling model no: 207, as a transmitter transducer and an accelerometer, model AMP01, as a receiver to measure the change in time of propagation of the elastic wave. Instead of using a zero cross detecting system, as used in U.S. Pat. No. 5,456,114, a cross-correlation detecting system is used. In operation, the system initially takes a base line initial reading of the known liquid level and stores this in a cross-correlator. The time of the propagation of an elastic wave along the propagation path is determined as a function of both the degree of the time shift between the signals and amount of sample by sample amplitude difference between signals. The limitation of this system is that every tank or vessel requires initial conditions and data comparison requirements for each application. This becomes impractical to implement, for example, in commercial uses such as for vessels of large liquid volume, sometimes a million or more gallons in a storage tank, which cannot be easily emptied or filled. Also, the mechanical shaker requires heavy duty electrical power to drive the mechanical transmitting transducer. This gives rise to installation problems since the storage tank can contain gasoline, diesel fuel or various hazardous chemicals. In such cases drilling and welding of the tank is not allowed.
The system of each of U.S. Pat. Nos. 5,456,114 and 6,192,751 uses a tone burst of 10 or more cycles of a high power driving signal to drive the transmitting transducer and also use the lowest order asymmetric flexural wave. This can reduce the accuracy of the measurement.
To overcome the above mentioned problems, the present invention provides a non-invasive level measuring system utilizing a pair of ultrasonic transducers mounted spaced apart externally on the vessel wall. One of the transducers, the transmitter, excites a single pulse of sonic (electro-mechanical) energy, which can be of relatively low amplitude. The second transducer is located at a fixed distance from the first and acts as a receiving transducer. The single pulse that is transmitted by the transmitting transducer in the vessel wall generates a flexural, or elastic, wave in the wall.
The system and method utilizes a component of the received flexural wave to detect presence or absence of liquid inside of the vessel in the space between the two transducers. This is accomplished using a technique that measures the change in phase velocity of the elastic wave as it travel through the vessel wall. That is, for any given vessel a component of the flexural wave produced by the pulse travels in its wall at a different velocity depending upon whether there is liquid in the vessel in the space between the two transducers. Measuring the difference, or phase delay, between the phase velocity of the flexural wave components when there is no liquid in the interior vessel space between the two transducers and when there is liquid present produces an indication of whether or not there is liquid in the space. Also, it has been determined that a substantially linear relation exists for the amount of the phase delay between the received components as to the height of the liquid level in this space. Therefore, the system can produce a continuous level determination of the liquid in the space or indicate that the liquid has reached one or more predetermined points.